Fix regression with multiple env substitutions for the same key

docs/changelog/2869.bugfix.rst

@@ -0,0 +1,5 @@
+Fix regression introduced in 4.3.0 which occured when a substitution expression
+for an environment variable that had previously been substituted appears in the
+ini file after a substitution expression for a different environment variable.
+This situation erroneously resulted in an exception about "circular chain
+between set" of those variables - by :user:`masenf`.

src/tox/config/loader/ini/replace.py

@@ -178,29 +178,32 @@ def join(self, value: MatchArg) -> str:
         return "".join(self(value))
 
     def _replace_match(self, value: MatchExpression) -> str:
+        # use a copy of conf_args so any changes from this replacement do NOT
+        # affect adjacent substitutions (#2869)
+        conf_args = self.conf_args.copy()
         of_type, *args = flattened_args = [self.join(arg) for arg in value.expr]
         if of_type == "/":
             replace_value: str | None = os.sep
         elif of_type == "" and args == [""]:
             replace_value = os.pathsep
         elif of_type == "env":
-            replace_value = replace_env(self.conf, args, self.conf_args)
+            replace_value = replace_env(self.conf, args, conf_args)
         elif of_type == "tty":
             replace_value = replace_tty(args)
         elif of_type == "posargs":
-            replace_value = replace_pos_args(self.conf, args, self.conf_args)
+            replace_value = replace_pos_args(self.conf, args, conf_args)
         else:
             replace_value = replace_reference(
                 self.conf,
                 self.loader,
                 ARG_DELIMITER.join(flattened_args),
-                self.conf_args,
+                conf_args,
             )
         if replace_value is not None:
             needs_expansion = any(isinstance(m, MatchExpression) for m in find_replace_expr(replace_value))
             if needs_expansion:
                 try:
-                    return replace(self.conf, self.loader, replace_value, self.conf_args, self.depth + 1)
+                    return replace(self.conf, self.loader, replace_value, conf_args, self.depth + 1)
                 except MatchRecursionError as err:
                     LOGGER.warning(str(err))
                     return replace_value

tests/config/loader/ini/replace/test_replace_env_var.py

@@ -6,7 +6,7 @@
 import pytest
 
 from tests.config.loader.ini.replace.conftest import ReplaceOne
-from tox.pytest import MonkeyPatch
+from tox.pytest import LogCaptureFixture, MonkeyPatch
 
 
 def test_replace_env_set(replace_one: ReplaceOne, monkeypatch: MonkeyPatch) -> None:
@@ -75,6 +75,20 @@ def test_replace_env_missing_default_from_env(replace_one: ReplaceOne, monkeypat
     assert result == "yes"
 
 
+def test_replace_env_var_multiple(replace_one: ReplaceOne, monkeypatch: MonkeyPatch) -> None:
+    """Multiple env substitutions on a single line."""
+    monkeypatch.setenv("MAGIC", "MAGIC")
+    monkeypatch.setenv("TRAGIC", "TRAGIC")
+    result = replace_one("{env:MAGIC} {env:TRAGIC} {env:MAGIC}")
+    assert result == "MAGIC TRAGIC MAGIC"
+
+
+def test_replace_env_var_multiple_default(replace_one: ReplaceOne) -> None:
+    """Multiple env substitutions on a single line with default values."""
+    result = replace_one("{env:MAGIC:foo} {env:TRAGIC:bar} {env:MAGIC:baz}")
+    assert result == "foo bar baz"
+
+
 def test_replace_env_var_circular(replace_one: ReplaceOne, monkeypatch: MonkeyPatch) -> None:
     """Replacement values will not infinitely loop"""
     monkeypatch.setenv("MAGIC", "{env:MAGIC}")
@@ -97,12 +111,34 @@ def avoid_infinite_loop() -> None:  # pragma: no cover
 
 
 @pytest.mark.usefixtures("reset_env_var_after_delay")
-def test_replace_env_var_circular_flip_flop(replace_one: ReplaceOne, monkeypatch: MonkeyPatch) -> None:
+def test_replace_env_var_circular_flip_flop(
+    replace_one: ReplaceOne,
+    monkeypatch: MonkeyPatch,
+    caplog: LogCaptureFixture,
+) -> None:
     """Replacement values will not infinitely loop back and forth"""
     monkeypatch.setenv("TRAGIC", "{env:MAGIC}")
     monkeypatch.setenv("MAGIC", "{env:TRAGIC}")
     result = replace_one("{env:MAGIC}")
     assert result == "{env:MAGIC}"
+    assert "circular chain between set env MAGIC, TRAGIC" in caplog.messages
+
+
+@pytest.mark.usefixtures("reset_env_var_after_delay")
+def test_replace_env_var_circular_flip_flop_5(
+    replace_one: ReplaceOne,
+    monkeypatch: MonkeyPatch,
+    caplog: LogCaptureFixture,
+) -> None:
+    """Replacement values will not infinitely loop back and forth (longer chain)"""
+    monkeypatch.setenv("MAGIC", "{env:TRAGIC}")
+    monkeypatch.setenv("TRAGIC", "{env:RABBIT}")
+    monkeypatch.setenv("RABBIT", "{env:HAT}")
+    monkeypatch.setenv("HAT", "{env:TRICK}")
+    monkeypatch.setenv("TRICK", "{env:MAGIC}")
+    result = replace_one("{env:MAGIC}")
+    assert result == "{env:MAGIC}"
+    assert "circular chain between set env MAGIC, TRAGIC, RABBIT, HAT, TRICK" in caplog.messages
 
 
 @pytest.mark.parametrize("fallback", [True, False])